Cells respond to DNA damage by activating signal transduction pathways that lead to cell cycle arrest, DNA repair and apoptosis. DNA damage checkpoints are essential for the maintenance of genomic integrity. The importance of the DNA damage response in human pathology, especially cancer, is well established. Mutations in various genes encoding DNA damage checkpoint proteins are associated with a variety of human disease syndromes and these are often characterized by an increased predisposition to cancer. Although significant progress has been made understanding DNA damage checkpoints, the network of pathways that compose the DNA damage response is complex and additional components remain to be identified. We have screened mutant zebrafish embryos for DNA damage response defects. This identified two mutant lines that fail to arrest mitotic entry following exposure to ionizing radiation. Each of these lines carries a mutation in a gene that we have named mail (mitosis after irradiation 1). The mail gene encodes a protein with orthologues in other vertebrate species, including humans. There are blocks of amino acids that are highly conserved across these species, but no motifs or domains to suggest a potential biological function. In addition to a checkpoint phenotype, unirradiated mail zebrafish have a defect in normal cell cycle progression. The goal of this proposal is to determine Mail's role in cell cycle regulation and DNA damage checkpoints. There are two specific aims: (1) to investigate the DNA damage checkpoint and cell cycle defects of mail zebrafish mutants;and (2) to establish the function of Mail in human cells.